Method for applying samples to be analyzed by gas chromatography and sampling tube

ABSTRACT

The invention relates to a method for applying samples to be analyzed by gas chromatography using a sampling tube, which has a section which contains the adsorbing agent and narrows essentially conically towards a bore which is free of adsorbing agent and has a reduced diameter, in such a manner that the flow rate of a gas which is guided through towards the narrowed section on the outlet side is at least approximately equal to the inlet-side flow rate, liquid or gaseous starting sample material being guided through the sampling tube towards the narrowed section, so that substances to be analyzed are adsorbed on the adsorbing agent, the sampling tube being flushed with drying gas, which is guided through the sampling tube in the same direction as the starting sample material, until the liquid phase, which is contained in the sampling tube, or the starting sample material is essentially removed, and the adsorbed sample being desorbed by means of a carrier gas which is introduced through the sampling tube from the narrowed side.

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for analyzing samples bygas chromatography using a sampling tube.

German Patent DE 44 19 596 C1 has disclosed a thermal desorption devicefor a gas chromatograph with a temperature-controlled furnace which isprovided with a carrier-gas connection and with an evaporator tube, theevaporator tube being an exchangeable sampling tube which can bereceived by a receiving chamber of the furnace and one end of which canbe inserted into a receiving piece, sealed with respect to the outsideby means of a seal, in which case the receiving piece has two seals,which are arranged spaced apart in the region of the casing, and can beinserted into a receptacle of corresponding design, the carrier-gasconnection opening in the region of an annular gap between receptacleand receiving piece, between the two seals situated on the casing, whichfor its part is connected to an annular gap surrounding the inserted endof the sampling tube, with a trap connected downstream. In this case,the sampling tube may contain either a solid or volatile orsemi-volatile substances adsorbed on an adsorbing agent as the sample.However, the sampling tube is not suitable for applying substances to beanalyzed by gas chromatography which are contained in a liquid orliquid-vapor-laden gaseous starting sample material, since liquidcontained in the starting sample material would freeze in the downstreamtrap and would block the gas chromatograph.

European patent application EP 0 245 642 A1 has likewise disclosed amethod for sampling by means of thermal desorption using a samplingtube, in which method adsorbed substances are desorbed on a capillarycolumn of a gas chromatograph by means of a carrier gas via an injectorcontaining the sampling tube.

The object of the invention is to provide a method and sampling tubewhich make it possible to analyze substances originating from liquid orliquid-vapor-laden gaseous starting sample material with as little lossas possible and without freezing.

SUMMARY OF THE INVENTION

A sampling tube is used which has a section which contains the adsorbingagent and narrows essentially conically to a bore, which is free ofadsorbing agent and has a reduced diameter, in such a manner that theflow rate of a gas which is guided through towards the narrowed sectionon the outlet side is at least approximately equal to the inlet-sideflow rate. This makes it possible to adsorb on the adsorbing agent, fromliquid or gaseous starting sample material, in particular in the form ofan aqueous liquid or an essentially liquid-vapor-saturated gas phase,the substances of the sample in the sampling tube which are to beanalyzed, by guiding the liquid or gaseous starting sample through thesampling tube towards the narrowed section, and to dry the adsorbedsample, by flushing with a drying gas, which is guided through thesampling tube in the same direction as the starting sample material,until the liquid phase, which is contained in the sampling tube, of thestarting sample material is essentially removed. The sampling tube isthen inserted into a thermal desorption device of a gas chromatographand the adsorbed sample is desorbed by means of a carrier gas which isintroduced through the sampling tube from the narrowed side.

As a result, it is possible to analyze in particular aqueous liquids,for example beer, wine, milk, drinking water, waste waters, ground wateror the like, by gas chromatography in a simple manner and, for examplewith continuous sampling. However, this method can also be used toanalyze substances contained in organic solvents.

There is virtually no occurrence of contamination which would beintroduced in the event of desorption using solvent, since gases usedfor desorption can be kept very pure.

Further configurations of the invention are to be found in the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an enlarged view of a sampling tube.

FIG. 2 diagrammatically shows a device for charging a sampling tube fromFIG. 1.

FIGS. 3a and 3b diagrammatically show two further devices for charging asampling tube from FIG. 1.

FIG. 4 shows a thermal desorption device for a sampling tube from FIG.1.

FIG. 4a is an enlarged view of a portion of the thermal desorptiondevice of FIG. 4.

FIG. 4b is an enlarged view of a portion of the thermal desorptiondevice of FIG. 4.

DETAILED SPECIFICATION

The sampling tube 1 illustrated in FIG. 1 has a through-bore 2 with aregion 3 which is open at one end, has a large diameter and narrows viaan essentially conical section 4 towards a narrowed region 5 with asmall diameter. The region 3 and the conical section 4 accommodate anadsorbing agent 6, which may optionally be situated on a carriermaterial. The diameters of the regions 3 and 5 are matched to oneanother in such a manner that the flow rate of a gas guided throughtowards the narrowed region 5 in this narrowed region is at leastapproximately equal to the flow rate in the widened region 3 which isfor its part narrowed by the adsorbing agent. That is, the reduced openarea through the adsorbing agent in the widened region substantiallyequals the area through the reduced diameter region. In this way,virtually all the liquid which is introduced during the charging of thesampling tube 1 can be flushed out using a drying gas, such as nitrogenor a noble gas (helium), without the formation of dead zones in whichconsiderable quantities of liquid remain.

The sampling tube 1 expediently consist of glass, although othermaterials, such as stainless steel, may also be used, and expedientlyhas a constant external diameter in order to allow simple, optionallyautomatic, handling.

In accordance with FIG. 2, a magazine 7 for sampling tubes 1 isprovided, a sampling tube 1 being situated in the charging position.This sampling tube 1 is positioned in such a manner that the widenedregion 3 is situated on the inlet side and is connected to a line 8. Theline 8 is connected, on the one hand, to a bypass injector 9 and, on theother hand, to a drying-gas source 11 via a valve 10.

The bypass injector 9 comprises a receiving tube 12 for starting samplematerial, which in the position shown in solid lines flows through thebypass injector 9 and its receiving tube 12. The receiving tube 12 is,for example, a stainless steel tube of predetermined length andpredetermined diameter and hence of predetermined receiving volume,which is expediently wound up into a spiral.

After charging the receiving tube 1, the bypass injector 9 switches overinto the position shown in dashed lines, in which a pump 13 forces aconveying liquid, for example distilled water, into the bypass injector9, with the result that the starting sample material is forced out ofthe receiving tube 12 and is guided, via the line 8, through thesampling tube 1, where substances contained in the starting samplematerial are adsorbed on the adsorbing agent 6.

Using the bypass injector 9 permits a distortion-free application of thestarting sample material, so that memory effects are avoided. Withstarting sample materials where there is no risk of such effects, theapplication may also be carried out directly, without the interventionof the bypass injector 9.

When the starting sample material has been guided through the samplingtube 1, the drying-gas source 11 is connected up via the valve 10 andguides drying gas through the sampling tube 1 in the same direction asthe starting sample material until the sample taken from this materialhas practically been dried. This can be monitored by means of a thermalconductivity detector 14, which on a suitable output signal ends thesupply of the drying gas.

In accordance with FIG. 3a, the sampling tube 1 may also be charged byconnecting it to a line 8' which opens into a bottle-like sample vessel15 above the liquid level of a liquid contained therein as startingsample material. Furthermore, a line 16 for extraction gas, such asnitrogen, air or the like, leads into the optionally heatable samplevessel 15, which line 16, below the liquid level, opens onto anopen-pore body which forms gas bubbles. Rising gas bubbles are laden orsaturated both with substances to be analyzed and with liquid vapor, sothat liquid is also introduced into the sampling tube 1. This liquid isremoved from the sampling tube 1 in the drying step.

In accordance with FIG. 3b, the line 16 may open out above the liquidlevel of the liquid situated in the sample vessel 15, while the line 8'reaches as far as the bottom of the sample vessel 15. As a result, theliquid situated in the sample vessel 15 can be forced out of the line16, by the gas, directly into the sampling tube 1, in which case, aftertermination of the transfer of the liquid, the gas is used for drying.In order to leave as little liquid remaining in the sample vessel 15 aspossible, the sample vessel 15, as indicated in FIG. 3b, may have adepression on the bottom side or may be of approximately conical designon the underside.

A suitable liquid is generally a solvent, in particular water, or asolvent mixture, e.g. water and alcohol, for the substances to beanalyzed.

In accordance with FIG. 4, the charged sampling tube 1 is inserted intoa thermal desorption device 17 of a gas chromatograph, as is essentiallyknown from DE 44 19 569 C1, with the widened region 3 as the inward end,so that desorption is carried out counter to the charging direction. Thethermal desorption device 17 comprises a furnace 18 which can be cooledand heated in a controlled manner and has an inner receiving chamber 19,which is surrounded, on the one hand, by a heating device 20a for thecontrolled heating of a receiving chamber 19 and, on the other hand, bycoolant bores 20b, which are connected to a coolant source, for examplea liquid-nitrogen source.

The receiving chamber 19 serves to receive in an exchangeable manner asampling tube 1, for which a receiving piece 21 is provided, which hasan axially central blind bore 22 which is provided, next to its opening,with a groove which accommodates an O-ring 23. The outside of thereceiving piece 21 is of conical design and is provided with two groovesarranged spaced apart on its conical surface, each of which accommodatesan O-ring 24. The blind bore 22 accommodates one end of the samplingtube 1 in an exchangeable manner.

At its end which is open towards the outside, the receiving chamber 19has a frustoconical receptacle 25 for the receiving piece 21 and, withthe sampling tube 1, forms an annular gap 26a. With the receiving piece21 inserted, the O-rings 24 seal off a section 27a of an annular gap 27between the receiving piece 21 and the receptacle 25, into which sectiona carrier-gas line 28 opens. That region of an annular gap 29,surrounding the sampling tube 1 in the blind bore 22, which is sealedwith respect to the receiving chamber 19 by the O-ring 23 is connectedvia a bore 30 to the section 26b of the annular gap 29 and hence to thecarrier-gas line 28, so that carrier gas supplied via the carrier-gasline 28 can flow into the sampling tube 1.

A transfer capillary 31, which leads to a trap 32, opens into thereceiving chamber 19, inside the sampling tube 1. The transfer capillary31 projects to some extent into the receiving chamber 19, so that whenthe sampling tube 1 is introduced it is to this extent accommodated bythe latter, forming an annular gap 32a.

A suitable trap 32 is, for example, a sample application device as isdescribed in EP 0 451 566 A1. This device comprises a head 33 with anevaporator tube 34 arranged therein, which evaporator tube can be cooledand heated in a controlled manner by means of a suitable cooling andheating device 35, 36. A temperature sensor (not shown) and a controldevice, which is not shown and if appropriate also regulates thecontrolled heating of the furnace 18 by means of a correspondingtemperature sensor, is provided for this purpose.

The evaporator tube 34 may be unfilled and provided with vortexinginserts or may be filled with an adsorbing agent, such as glass wadding,or may be provided with an inner coating or filled with anapplication-dependent adsorbing agent.

A pneumatic, ventilated closure 39, which is connected to thecarrier-gas source, is provided at the outlet end of the vaporator tube34, into which a gas-chromatographic capillary 38 is introduced, formingan annular gap. When the closure 39 is opened, carrier gas flows pastthe capillary 38, through the evaporator tube 34 and the transfercapillary 31, into the sampling tube 1. The carrier gas flows furtherthrough the annular gap 19 and a discharge line with a correspondinglyopen changeover valve 37. The carrier gas flowing through the samplingtube 1 is thus prevented from penetrating into the trap 32. Rather, inthis way liquid which may have been adsorbed on the adsorbing agent oron the substances adsorbed thereon can be desorbed and removed from thesystem. This second drying step may, if appropriate, be carried out atelevated temperature in order simultaneously to remove high-boilingsubstances.

The thermal desorption of the substances of the sample in the samplingin the sampling tube 1 which are to be analyzed takes place by guidingcarrier gas through the sampling tube 1, from its end with the narrowerregion 5 to the wider region 3, the thermally desorbed substances beingcollected in the trap 32. The thermal desorption device 17 in this caseoperates with split (changeover valve 37 in the position illustrated),while the trap 32 is without split. The substances are then applied tothe capillary 38, the trap 32 operating with or without split.

The transfer capillary 31 is arranged in a transfer chamber 40, whichcontrols the temperature of the capillary and extends from the furnace18 to the head 33, in order to avoid losses of substance as a result ofcondensation.

The carrier gas may also serve as a flushing gas between the individualthermal desorptions.

Depending on the concentration of the substances to be analyzed, it ispossible to operate the thermal desorption device 17 and also the trap32 with or without split.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof, and it istherefore desired that the present embodiment be considered in allrespects as illustrative and not restrictive, reference being made tothe appended claims rather than to the foregoing description to indicatethe scope of the invention.

We claim:
 1. Method for applying samples to be analyzed by gas chromatography using a sampling tube, the substances to be analyzed of the sample being adsorbed by an adsorbing agent arranged in the sampling tube and the sampling tube being inserted into a thermal desorption device of a gas chromatograph for desorbing the substances to be analyzed, the method comprising the steps of:a) utilizing a sampling tube which has an inlet side with a first section which contains the adsorbing agent extending across the cross-section of said first section, the sampling tube narrows essentially conically to an outlet side with a narrowed second section with a bore which has a reduced diameter and is free of the adsorbing agent and has a reduced diameter, the sampling tube configured such that when a gas is guided from the inlet side towards the outlet side, the flow velocity of the gas at the narrowed second section on the outlet side is substantially the same or greater than the velocity of the gas at the first section; b) guiding a liquid or gaseous starting sample material, in particular in the form of an aqueous liquid or an essentially liquid-vapor-saturated gas phase, through the sampling tube from the inlet side towards the narrowed second section, so that substances to be analyzed are adsorbed on the adsorbing agent; c) flushing the sampling tube with drying gas through the sampling tube in the same direction as the starting sample material, until the liquid phase, which is contained in the sampling tube, or the starting sample material is substantially removed; and d) introducing through the sampling tube from the outlet side a carrier gas whereby the adsorbed sample is desorbed.
 2. Method according to claim 1, further comprising monitoring the drying by a thermal conductivity detector.
 3. The method according to claim 1, further comprising a further drying of the sample in the thermal desorption device by means of a carrier gas prior to the desorption.
 4. The method according to claim 3, wherein the further drying is performed at an elevated temperature.
 5. The method according to claim 3, further comprising pneumatically closing a trap of the thermal desorption device during the further drying.
 6. The method as in claim 1, wherein the starting sample material is initially taken up by a bypass injector and is then guided through the sampling tube.
 7. The method as in claim 1, wherein the starting sample material is transferred into the sampling tube from a sample vessel by means of gas.
 8. The method of claim 5 wherein the starting sample material is initially taken up by a bypass injector and is then guided through the sampling tube.
 9. The method of claim 5 wherein the starting sample material is transferred into the sampling tube from a sample vessel by means of gas.
 10. A method for applying samples to be analyzed by a gas chromatograph with a thermal desorption device capable of receiving a sampling tube, the method comprising the steps of:a) utilizing a sampling tube which has an inlet side with a first section which contains the adsorbing agent and narrows essentially conically to an outlet side with a narrowed second section with a bore which has a reduced diameter and is free of the adsorbing agent and has a reduced diameter, the sampling tube configured such that when a gas is guided from the inlet side towards the outlet side, the velocity of the gas at the narrowed second section on the outlet side is at least approximately equal to the velocity of the gas at the first section; b) guiding a liquid or gaseous starting sample material through the sampling tube from the inlet section towards the narrowed section, so that substances to be analyzed are adsorbed on the adsorbing agent; c) flushing the sampling tube with drying gas through the sampling tube in the same direction as the starting sample material, until the liquid phase, which is contained in the sampling tube, is essentially removed; d) inserting the sampling tube into the thermal desorption device; and e) introducing through the sampling tube from the outlet side a carrier gas whereby the adsorbed sample is desorbed by thermal desorption.
 11. Method according to claim 10, further comprising monitoring the drying by a thermal conductivity detector.
 12. The method according to claim 10, further comprising a further drying of the sample in the thermal desorption device by means of a carrier gas prior to the desorption.
 13. The method according to claim 12, wherein the further drying is performed at an elevated temperature.
 14. The method of claim 12, further comprising pneumatically closing a trap of the thermal desorption device during the further drying.
 15. The method as in claim 10, wherein the starting sample material is initially taken up by a bypass injector and is then guided through the sampling tube.
 16. The method as in claim 10, wherein the starting sample material is transferred into the sampling tube from a sample vessel by means of gas.
 17. The method of claim 14, wherein the starting sample material is initially taken up by a bypass injector and is then guided through the sampling tube.
 18. The method of claim 14 wherein the starting sample material is transferred into the sampling tube from a sample vessel by means of gas.
 19. The methodology of claim 10 wherein the starting sample material is in the form of an aqueous liquid.
 20. The methodology of claim 10 wherein the starting sample material is in the form of an essentially liquid-vapor-saturated gas phase. 